1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for producing such a magnetic recording medium.
2. Discussion of the Related Art
In recent years, precision working techniques, particularly ultra-smooth working techniques, for substrates made of brittle materials such as ceramics and carbon are in demand. For example, substrates, such as glass substrates, carbon substrates, and ceramic substrates, are used as substrates for magnetic recording media. In addition, silicon wafers are used in materials for semiconductors. Since the substrates for magnetic recording media and the silicone wafers mentioned above are required to have good ultra-smoothness, the smoothening of these substrates is generally achieved by polishing with loose abrasive grains. As the recording densities and memory capacities increase, a demand for the smoothness is escalating every year. Today, the arithmetic mean deviation of the profile Ra in the order of 10 .ANG. or less, in some cases 5 .ANG. or less, is in demand.
In the case where polishing of particularly brittle materials, such as glass substrates, carbon substrates, ceramic substrates, and silicon wafers, is conducted with loose abrasive grains, microcracks are liable to be generated during polishing owing to the brittleness of the substrates. Such microcracks are liable to induce read-write errors when the substrate is prepared into a magnetic recording medium or to corrode the substrates owing to foreign contaminants entering upon polishing or water formed upon capillary condensation while allowing to stand. Therefore, in view of suppressing the generation of the microcracks, conventionally, the polishing processes have been generally separated into multi-steps while sacrificing productivity. Since the polishing with loose abrasive grains has quite a highly artistic, skilled element, highly skilled techniques are required for achieving quality stabilization. Therefore, in order to obtain a mirror-polished substrate having a small Ra by the conventional methods of producing magnetic recording media, such factors as time, complicated procedures, and skilled techniques must be involved.
Further, aside from the above problems, the above methods have a problem in which a part of the abrasive grains remains in the substrate after polishing. The complete removal of the residual abrasive grains substrate is difficult even if the substrates were subjected to cleaning. When a sputtered, thin film is formed on a substrate still having residual abrasive grains, the breakages are generated in the formed film owing to the presence of the residual abrasive grains, thereby resulting in damages in the magnetic head. Also, in the cases where the above substrates are subjected to polishing with loose abrasive grains, uneven polishing ascribed to unevenness in the materials used is caused, i.e. shallow pits are formed because portions more easily polished are deeply scraped off.
In order to solve the problems mentioned above, various studies have been conducted to smoothen the substrate without loose abrasive grains. As one method, a method of smoothening the surface of the substrate by grinding the surface by a ductile-mode grinding with fixed abrasive grains has been proposed (see Hara et al., Proceeding of the Japan Society for Precision Engineering (1992), Autumn, pages 19 and 20). It is more preferable to conduct grinding of the brittle materials such as carbon substrates and ceramic substrates by ductile-mode grinding, because the following effects can be achieved.
(1) Since the extent of depths of cut of the individual abrasive grains in the substrate can be set to a level equal to or less than the ductile-brittle transition point of the substrate, the working mode of the substrate can be controlled based on ductile (plastic) deformation or ductile removal rather than brittle fracture, thereby making it possible to suppress the generation of microcracks; PA1 (2) Since no loose abrasive grains are used, there are no danger of having residual abrasive grains on the substrate; PA1 (3) The resulting substrate has excellent flatness, and having smaller amounts of rounding at edge portions normally observed in those obtainable by grinding with loose abrasive grains; PA1 (4) The wear of the fixed abrasive grains is only slight, thereby substantially lowering the costs required for tools of consumption when compared with a case where disposable, loose abrasive grains are used; PA1 (5) There are little artistic, skilled elements, thereby making process management and full-automation easy; and PA1 (6) The resulting surfaces after grinding are made even regardless of the unevenness of the materials used. PA1 (a) subjecting surfaces of a substrate to grinding by a ductile-mode grinding with a grinding wheel having a setting depth of cut of from 0.05 to 20 .mu.m, to give a surface-worked substrate having grinding marks of arc patterns; and PA1 (b) forming recording medium-constituting layers at least comprising an under layer, a magnetic layer, and a protective layer on the surface-worked substrate, or on a mirror-polished substrate obtained by further subjecting the surface-worked substrate to a finish polishing. PA1 (a) a surface-worked substrate having grinding marks of arc patterns or a mirror-polished substrate obtained by subjecting the surface-worked substrate to a finish polishing; and PA1 (b) recording medium-constituting layers at least comprising an under layer, a magnetic layer, and a protective layer,
However, in the method of Hara et al., the grinding is carried out by an ultraprecision grinding machine manufactured by CUPE equipped with a cup-type wheel, the grinding marks are in multiple, crisscross patterns as shown in FIG. 1. In these grinding marks, the marks intersect with one another at numerous locations, and at these intersected points microcracks may undesirably take place. Also, density differences in the grinding marks appear in radial directions, thereby causing to have an unevenness in the distribution of residual internal stresses, or in the distribution of the work damaged layer on the surfaces of inner diameter side and the outer diameter side. Therefore, the physico-chemical properties of the surfaces are uneven, thereby causing differences in etching properties of the surfaces or in adhesion and corrosion resistance when forming thin films, which in turn causes warp in the resulting substrates.
Also, in the case where the grinding is carried out by a double-sided polishing machine with fixed abrasive grains, the grinding marks are in random crisscross patterns and the marks intersect with one another at numerous places, thereby undesirably generating microcracks. Further, in cases of subjecting the substrates to surface grinding by a machine with a cup-type wheel, the workpiece and the working surface of the grinding wheel are surface-contacted with each other. Therefore, the feeding direction of cut is set perpendicular to the contacted surface, so that grinding resistance in the direction of the depth of cut becomes excessive, thereby making it liable to cause damages on the workpieces and the grinding wheels, and also that deep scratch damages and microcracks on the workpiece are caused owing to the breaking down of the abrasive grains. Therefore, in order to obtain a mirror-polished substrate by the above method, it would be necessary to carry out polishing in the subsequent polishing process until the above scratching damages are completely removed. Thus, the finish polishing becomes time consuming, thereby resulting in providing limitations in improvements of productivity of the magnetic recording media.